The study in the January 2 edition of the Journal of Clinical Investigation was led by a team of scientists at Washington University School of Medicine in St Louis, Missouri.

Subjects were encouraged to eat 1,000 extra calories per day, mainly by frequenting fast food restaurants with the goal of adding six percent of their body weight.

"This was not easy to do. It is just as difficult to get people to gain weight as it is to get them to lose weight," said chief author Elisa Fabbrini, assistant professor of medicine.

Those not suffering from ailments typically associated with obesity at the outset of the study -- such as insulin resistance, high cholesterol, high blood pressure and excess liver fat -- did not have these problems even after adding about 15 pounds (seven kilograms) to their already overweight frames, said the findings.

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The results mirror what scientists have seen in the general population, which is that about a quarter of obese people do not appear to suffer from metabolic complications that can lead to heart attack, diabetes and stroke.

But those whose did suffer from metabolic problems before the study grew even worse when they put on extra weight.

- Fat-regulating genes -

All the research subjects were helped by dieticians to lose the weight they gained for the study, which was featured on HBO's documentary "Weight of the Nation" while the research was under way.

Study authors said they now understand better how to distinguish obese people who will be more prone to ill health from those who may be more protected.

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For instance, those with abnormal metabolisms accumulated fat in their livers, while the healthier people did not, even when they gained more weight.

"Another difference involved gene function in fat tissue," the university said in a statement.

The study found that overweight people with normal metabolisms had more fat-regulating genes.

"The activity of those genes increased even more when the metabolically normal people gained weight. That wasn't true for people with abnormal metabolism," the study found.

Since obesity is known to contribute to dozens of health problems, more research is needed to figure out why some are particularly prone to problems from obesity.

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"Could it be genetics, specific dietary intake, physical lifestyle, emotional health or even the microbes that live in the gut?" queried senior investigator Samuel Klein, director of Washington University's Center for Human Nutrition.

"We need more studies to try to understand why obesity causes specific diseases in some people but not in others," Klein said.